Power transmission system



July 4, 1933. H. E. STOKES POWER TRANSMISSION SYSTEM 2 Sheets-Sheet 1 Filed June 22, 1929 R O T N E V N HaraZdE. Sta/res.

' ATTORNEY July 4, 1933. H E. STOKES POWER TRANSMISSION SYSTEM Filed June 22, 1929 2 Sheets-Sheet 2 m com gag 'INVENTOR Harold E. StoKe 5 Patented July 4, 1933 UNITED STATES PATENT orrlce HAROLD E. STOKES, OF MURRYSVILLE, PENNSYLVAN 1A,,ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA,

POVJER TRANSMISSION SYSTEM 'i Application filed June 22,

My invention relates generally to powertransmission systems and more part cularly to powersystems suitable for operating rolling mills and machines of similar nature wherein each roll or element of the mill or machine is driven by a: separate motor.

In the operation of rolling mills, it is the usual practice to operate a pair of rolls, forming a single stand, by means of a single motor connected to both rolls through a suitable gear box having cooperating pinion members connected to the roll spindles. As a result of the increasing demand for more economical mill installations, the single-motor" drive has proved unsatisfactory, since the initial cost of the installatiomas well as the maintenance cost, is high.

The present day practice has extended rolling-nill operations to include many different rolling schedules. In order to meet these demands, the mill operator following the old practice must necessarily make a large investment to provide a suflicient number of mill rolls to maintain the required schedules. The reason for the great increase of rolls, with the increase of schedules, results, because the single-motor drive does not lend itself to relative speed variation of the rolls.

In other words, when a gear box having a fixed gear ratio is utilized, as in a singlemotor drive, the speed of the individual rolls is fixed, and rolls of different diameters can not be used interchangeably in order toreduce the number of roll sets required for the different schedules.

Furthermore, the natural tendency is for the mill rolls to wear, changing their respective. diameters and, consequently, causing their peripheral speeds to become unequal, unless a corresponding change is made in their speed ratio. Since the rolls in a singlemotor drive are mechanically connected through the gear box, which has a fixed gear ratio, there can be no readjustment of their relative speeds to maintain the peripheral speed of one roll exactly equal to that of the other unless a change be made in I the gear ratio of the drive.

Another undesirable feature resulting from worn rolls is that the metal which is being rolled tends to cause the rolls to operate at equal peripheral speeds which do not con-v form to the fixed speeds at which the posi tively connected motor tends to drive them. lVhen this condition occurs, stresses are set 1929. Serial No. 372,843.

motors individually connected to the separate rolls of a rolling mill.

Another object of my inventlon is to provide for readily adjusting the relative speeds of the cooperating rolls of a mill driven by separate motors to maintain equal peripheral speeds, regardless of the diameters of the re: spective rolls. j

Other obj ects of my invention will become evident from the following description, taken in conjunction with the drawings,in.which Figure 1 is a diagrammaticview of a power system suitable for application to rolling mills having individual-roll drive motors, embodying the principal features .of my invention.

' Fig. 2 is a diagrammatic View showing a modification of the power system of Fig. 1 which may be utilized for non-reversing mills, and v Fig. 8 isa simplified diagrammatic view of a rolling mill showing how individual-roll drive motors may be applied. 1

Referring now to the drawings, 10 and 11 designate, generally, direct-current motors of any suitable type which may be utilized for operating the rolls 12 and 13 of a rolling mill of a conventional type, as shown in Fig. 3.

While, in this disclosure of the invention, thepower system is applied to rolling-mill drives, it will become evident as the description proceeds that it may also be readily applied to other machines having similar operating characteristics.

In this embodiment of the invention, com.- pound-wound motors are utilized for driving the rolls. The motors are provided with main or separately-excited field windings 1 1' and 15, cumulative compounding field windings 16 and 17 and differential compounding field windings 18 and 19. In this instance, the compounding-field windings function in the same manner as ordinary series windings. However, they are of different con-- struction and the effect which .they produce may be more readily controlled and varied, as will be pointed out hereinafter.

i The roll motors 10 and 11 may be supplied with power from any suitable source of direct-current energy. However, in thisinstance, since it is desirable to control the speed and direction of operation of the motors by variable-voltage control, a plurality of'direct-current generators 2-1 and 22'are uti: lized. As shown, the generators are mechanically connected by a shaft 23 and electri-. cally connected in parallel-circuit relation to the bus bars 24 and 25, to which the control motors 10 and 11 are also connected in parallel-circuit relation.

The generator may mover of any suitable type, which, in this instance, is not shown, since the construction and operation of motor-generator sets'of this determined load balance between-the two machines. I Since generator connections of this kind are well'known, the functlon and operation of the SGl18S'W1I1Cl1I1gS, whenconnect-ed in this manner, will be described only generally. Themain' field, which determines the voltage of the generators, is developed bythe main or separately excited field windings 26 and 27. The .difierential series-field windings are connected to'establish a field flux in it the opposite direction to the flux established by the main'or separately-excited field windings, while cumulative series-field windings are connected to establish a field flux in the same direction as the main field. If the gen,- erators' are of equal capacity, the mainfields areregulated or adjusted to cause an equal divisionvof load between the two machines or, if the machines are of unequal capacity, the relative excitation of the main field windings may be adjusted to divide the load in the de sired ratio. I i

Since the cumulative series field windings are cross connected, they may be caused to exactly balance and counteract the excitation produced by the differential fields when the loads on the generators are equal or when the total load is being divided in a predetermined ratio. .I'Vhen the load becomes unbalfanced, the heavily-loaded generator decreases its mainEfield flux through the action of its differential series-field winding, which is in series with its armature, while the opposite eifect is produced upon the main-field flux of the other machine, since its cumulative series-field is cross connected to the be driven by a prime- In order to control the direction of the main field of the generators and, consequently, the voltage applied to the motors l0 and 11, a plurality of reversing switches 32 to 85, inclusive, are provided. 7

The speed of the. roll motors 10 and 11 may be controlled by either varying the voltage of the generators or by varying the field excitation of the motors. In this instance,

one point of'speed control by each method is shown, although it is to be understood that as many points of speed controlas des1red may be obtained. As shown, a field resistor 36, and a bridging switch 37, are provided in the energizing clrcuit of the generator main field windings 26 and 27 to vary the field current in a one-step operation. llhe speed control obtained by varying the excitation of the' motor fields will be described in detail, here:

inafter.

In orderto control the operation of the roll motors in accordance with the present invention, a plurality of series exciters 38 to 41, inclusive, are provided. The exciters are provided with field windings 42 to 45,

inclusive, connected in series with the motor armatures, as shown. i r

In this embodiment ;of the invention, one of the main functions of the'series exciters is to provide an energizing voltage for the compounding-field windings on the motors which shall vary in direct proportion to the load currents flowing in their respective circults. .Accord1ngly,the exciters are-driven I at substantially constant speed and operated on the lower portion o their saturation curve in order that a change in field excitation shall cause a' corresponding change of terminal voltage. Constant-speed motors of any suit. able type may be utilized for driving the ex citers and, in this instance, motors designated, generally, by the numerals 46,47 and 48 are provided.

In order'to control the direction of flow of current in the exciter circuits, 1a plurality of reversing switches 49 to 56, inclusive, are provided. As shown, a pair of switches are utilized in each exciter circuit, the function and operation of which will be described in detail hereinafter.

As described hereinbefore, the second step of speed control is obtained by varying the field excitation of the motors. Therefore, in order to vary the excitation ofthe main field windings 14 and 15, a field-resistor 57 and a bridging switch 58 are provided. In this instance also, a plurality of points of speed control may be obtained by simply providing a plurality of bridging switches for the resistor.

The operation of the system may be controlled in any suitable manner. However, in this embodiment of the invention, a manually-operated master switch 59, provided with drum se ments 60 and 61 and stationary I contact fingers 62 to 67, inclusive, is utilized.

In order that a full and complete understanding of my invention may be obtained, the operation of the system, as it is applied to a rolling mill having individual roll drive motors, will now be described in detail.

Assuming that the generators 21 and 22 are being driven by a prime mover (not shown), then a voltage for energizing the bus bars 2e and 25 may be developed. As shown,

a master switch 59 is provided for controlling the operation of the bridging switches 37 and 58, generator field reversing switches 32, 33, 3% and and motor field reversing switches 49 to 56, inclusive.

If it is desired to operate the roll motors 10 and 11, in the forward direction, the drum segment 60 of the master switch 59 may be actuated to the first position into engagement with the contact fingers 62, 63, 6st and 66.

An operating circuit is thereby established for actuating the proper motor and generator field-reversin g switches which may be traced 11'0111 the line conductor 70, through conductor 72, contact fingers 62 and 64 bridged by the segment 60-conductor 73,

operating coils of the motorfield reversing switches 56, 54, 52, and 50 and the operating coils of the generator field-reversing switches 33 and 35, to the line conductor 71.

In response to the closing of switches 33 and 35, an energizing circuit for the gen erator main-field windings is established which extends from the line conductor 70, through conductor 72, contact fingers'62 and 63bridged by the segment conductor 7% switch 33, conductor 75, main field windings 27 and 26, conductor 76, switch 35 and field resistor 36, to the line conductor 71.

In order to provide a strong shunt-field excitation for the roll motors l0 and 11, during the starting operation, provision is made for bridging the field resistor 57 during the initial starting period. As shown, an actuating circuit for the bridging switch 58 may be established which cXtends from the energized contact finger 66 through conductor 7 7 and operating coil of the switch 58, to the line conductor 71. Therefore, the switch 53 is closed when the drum 59 is actuated to its first position.

The speed of the roll motors 10' and 11 may be further increased by actuating the master switch to the second position, thereby energizing the contact fingers 6'? and establishing an operating circuit through the conductor 78 for the bridging switch 37, as shown.

Upon the closure of the switch 37, the generator-field resistor 36 is bridged, and the main field excitation of the generators increased a predetermined amount. Further increase in speed of the roll motors may be obtained by actuating the master switch to the third position, thereby deenergizing the contact finger 66 which interrupts the operating circuit of the bridging switch 58. When the switch 58 opens its contact members the field resistor 57 is reinserted in the excitation circuit of the main field windings 14 and 15. r

In performing rolling operations, it is desirable to rotate the rolls at the same peripheral speed, independently of their diameters. Therefore, it becomes necessary to provide for readily adjusting the initial speed of the roll motors in order to obtain equal peripheral speeds. Since the main fields predominate in controlling the operation of the motors, provision is made for separately varying the excitation of the main field windings let and 15. Field rheostats of any suitable type may be utilized for this purpose which, in this instance, are shown in diagrammatic form at 81 and 82. I

It will be readily understood that, through the agency of the rheostats, the initial relative speeds of the motors 10 and 11 may be adjusted at will. Thus, if the diameter of one roll is one half that of the other, the speed of its motor may be doubled and that of the other may remain unaffected or any other desired speed adjustment may be made.

When the metal is engaged by the rolls, an entirely difierent problem of control presents itself. In order to perform a desirable rolling operation and to obtain a desirable prod not, it is necessary that each roll shall exert the same pulling or reducing force upon the metal during the rolling operation. This is necessary in order to roll a flat strip or a flat object of any kind since, if the rolls rotate at different peripheral speeds,'the pulling forces, of reduction, applied by the rolls are unequal, and the reducing process will be unbalanced. When a condition of this kind ex ists, the reduction of one side of the metal will be carried on at a faster rate than on the other side, and, in leaving the rolls, it will naturally acquire a curved shape. Therefore, it is evident that the initial speeds of the roll motors 10 and 11, as determined by the tained by cross connecting the differentialcompounding-field windings 18 and 19 and maintaining balanced condition between the excitation produced by the compounding-field this field winding varies in direct proportion to the motor load. However, its differential field-winding 18 is cross-connected to the se ries exciter of the motor 11 and, therefore, its excitation varies in direct proportion to the load on the motor 11. The same scheme is used for the field windings 17 and 19 of the motor 11. Therefore, it is evident that the load currents in the motor circuits must remain at the'same relative value in order that a balanced condition may be maintained between the compounding fields of each motor.

In order that a more comprehensive under.- standing of the balancing operation may be obtained from the description which is to follow, the cause of load'unbalance will first-be explained.

In manufacturing motors and electrical machines of this character, it is difficult to produce motors which are exactly alike. There is always a slight variation in those parts which determine the operating characteristics of the motor, that is, the air gaps are unequal or the iron used in the magnetic circuit is not of the same quality or quant is evident that, when two motors differ in' such respects, their operating characteristics are not the same and, consequently, a seemingly correct adjustment of their shunt-- field currents does not cause them to divide the load in a predetermined ratio, when they operate in parallel-circuit relation and cooperate in carrying the same total load.

It will be readily understood that, if the motor 10, for example, becomes overloaded, for any reason, the current flowing through its armature circuit and the field windings 12 and 13 of the exciters 38 and 39 is increased.

The exciter voltages are immediately increased, thereby increasing the cumulative field excitation of the motor 10 and the differential field excitation of the motor 11. he effect of this is to strengthen the effect of the cumulative field of the motor 10 and to weaken its effect in motor 11.

Since the cumulative field aids the main field, the total field flux in the motor 10 is increased, causing its counter E. M. F. to

increase which opposes and decreases its load current to reduce the load.

' The increased excitation of the differentialfield winding 19 of the motor 11, however, produces exactly the opposite effect on this motor. Since its load does not change appreciably during the initiation of the balancing operation, the excitation of its cumulative-field winding 17 remains substantially constant. An increased excitation of the differential-field winding 19, however, interrupts the balanced-field condition in this motor, and, since the flux produced by the differential-field winding 19 is in opposition to that produced by the main field winding 15, the total excitation of the motor is reduced. When this condition exists, the counter electro-motive force of this motor is decreased and renders it more susceptible to assuming an increase in load current.

In view of the foregoing explanation, it is evident that the balancing force created by aiload unbalance is doubly effective. In the first instance of the heavily loaded motor, a condition is established which causes this motor to reduce its load, while, in the second instance, a condition is established in the.

other motor which causes it to assume a greater load. Tt is evident that the heavilyloaded motor is not merely prevented from taking more load, but conditions are set up in the lightly-loaded motor which cause it to actually take a part of the load of the heavily loaded motor.

The double effect of the balancing force may be further illustrated by assuming that the load on the motor 10 is sufficient to enable 1 it to produce, for example, +1500 ampere -turns in its cumulative-field winding 16,

which would cause the differential field winding 19 of the motor 11 to produce 1500 ampere turns, while the load motor 11 produces +1000 ampere turns in its cumulative-field winding 17 and, therefore, -1000 ampere turns in the differential-field winding 18 of the motor 10. WVhen this condition exists, the motor 10 is +500-ampere turns, while, in the motor 11, it is 500 ampere turns. Therefore, it is evident that the total unbal-' ance between the fields of the two motors is 1000 ampere turns, which is double the amount which could be produced unless the fields of both motors were differentially varied by utilizing cross-connected differentialfield windings. r

It will be readily understood that the load ratio of the motors is automatically maintained in response to the continuous opera tion of the balancing force and its effect upon the motor speeds. The balancing force in the system is created by the load unbalance and, consequently, it appears and continues to exist to cause the motors to always divide the unbalance in the field excitation of as would be the case and speed-maintaining effect.

relative speeds of the motors throughthe operation of the balancing force.

The importance otmaintaining a predetermined speed ratio between the motors is realized more fully when the effect of unbalanced counter electro-motive forces between the two motors is considered. Assuming that the excitation ofthe motors remains constant, their respective counter electro-motive forces vary in direct proportion to their speed, and, sincethey are not mechanically connected except by the metal'between the rolls, thereis a tendency for one motor to constantly run faster than the other. Since the loop circuit connecting the two motors is of very low resistance, a small unbalancing of the counter electro-motive forces causes an extremely high circulating current to flow between the motors. This condition'may occur when'motors of any capacity are connected and operated in this manner. How ever, the larger the motors the more serious the condition becomes, since only a small unbalance may cause the flow of several thou sand amperes of circulating current. 7

As set forth hereinbefore, each exciter circuit is provided with a pair of reversing switches which, as described, are utilized to maintain the proper direction of flow of the exciting current in each of the compounding-field windings. As is well understood, when the motors 10 and 11 are reversed by reversin the a) lied volta e a reversal of their armature current occurs. This causes the exciters to reverse their polarity and, consequently, the excitation of the compounding-field windings is reversed. Since the direction ofthe main field excitation is not changed, it is evident that the flux produced in the compounding-field windings must always he maintained in the same direction in order that the proper balancing eifect may be obtained. The use 'of field-reversing switches in the exciter circuit is only necessary in case the roll motors are to be reversed,

for a reversing mill;- As described in detail hereinbefore, it is necessary to utilize crossed-fieldcontrol. in the system in order to obtain a load-balancing 7 Furthermore, it the mill is of the reversing type using W'ard Leonard speed and direction control, it is necessary to reverse the excitation of the compounding-field windings inaccordance with each reversal of the motors. It is evident that it would be practically impossible to reverse the excitationin this manner if series field windings of the ordinary type were utilized tor the reasonthat they would carry the entire load current of the motor. This would require the use of high-capacity switches and acomplicated control. apparatus in order-to effect their reversal. I

However, it is evident that this function may be readily accomplished by utilizing the 86 is also energized to establish series-exciter scheme embodied in this invention. In this instance, the compoundingfield windings produce the same effect as ordinary series windings by utilizing a small excitation current which is, at all times, proeration of the series exciters, and it is easily controlled by utilizing reversing switches in the exciter circuits. p 7

In Fig. 2 of the drawings, is shown a power system embodying the same features of my invention suitable for'application to .70 portional' to the load current, due to the opment' 87 and contact fingers 88 to 91, inclusive, is provided for controlling the operation of the system.'. As shown, when the master switch is actuated to the first posi tion, contact fingers 88 to'89 are bridged by the segment 87 to establish an operating circuit for the field switch 85 which extends from the line conductor .71, through conductor 92, contact fingers 88 and 89-bridged by the segment 87operating coil of the switch 85 and conductor 93, ductor/ZO. 'Upon the closure of the field switch 85, the excitation circuit of the generator main field windings 26 and 27 is established. I

The contact fingerQ O of the master switch an operating circuit for the switch 58, the motor-field resistor 57 mum field excitation for and 11 during their starting operation. The speed of theroll motors 10 and 11 may be further increased by actuating. the master switch to the second position, thereby energizing the contact finger 91 and establishingan opto provide maxito the lineconthereby bridging the roll motors 10 erating circuit for the switch 37 which,'upon the generators. [The operation of the mas- 'ter switch tothe third position interrupts the operating circuit of the bridging switch 58, thereby rendering the field resistor 57 effective to decrease the excitation of the mo-.

tors to further increase their speed. 1

It will be readily understood that, since this system isintended' only for non-reversing operation, it is unnecessary to provide for reversing the excitation of the-compoundingfield windings of the motors 10 and 11 in order tomaintain the balancing effect inthe system. As shown, the dil'i'erential-field windings 18 and 19, in this instance, are in the form of regular series-field windingsconnected in the motor, circuits in series with I system are exactly I plurality siredto maintain a compounding field windings the field'windings lZ and 45 of the series exciters 38 and 41, respectively.

The method of connecting the compounding-fieldwindings oi the system in Fig. 2 may be exactly opposite that shown, if desired. That is,- the cumulative-field windings 16 and 1 may be in the torm of regular series wind- 'ings, and the differential field produced by compounding-field windings excited from the exciters. 1 v

' The latter method of excitation would be of particular advantage, in obtaining proper speed regulation of the motors, if it were. de-

constant operating speed for tandem-mill application. In this event, the series fields would bedesigned to give the motors a drooping-speed characteristic, and the excitation of the differential-com,- pounding field windings varied by means of suitable rhe'ostats to give the desired op'en ating speed for the load at which the motors were required to operate.

From the foregoing description, it is evident that the operation and the functions producedby the different elements of this the same as those produced by the system shown in Fig. 1. It may be'stated in conclusion that, while the illustrated example constitutes a practical embodiment of my invention, I do not wish to limit myself strictly to the exact details herein illustrated, since other modifications may be made without departing from the spirit of the invention, as defined in the appended claims.

'I claim as my invention:

1. In a power system, in combination, a of motors and generators, said motors being connected in parallel-circuit relation to said generators, each motor being provided with a main field winding and cumulative and difierential compounding field windings, and exciter means responsive to the load on any one motor for establishing an unbalance in the; field excitation of the or" all the motors, thereby to correct for a load unbalance between themotors, caused'by a variation intheir relative speeds.

2. In a'power system, inoOmbinat-ion, a

plurality of motors each provided with a mam field, cumulative compounding and differential compounding-field windings, means for individually energizing the differentialcompounding windings of each motor in accordance with the'current flowing in the othermot-or, and means for individually on ergizing the cumulative-field windings of each motor in accordance with the current flowing in that motor, to obtain balarce of load.

3. In a power system for motors dividing their loads in a predetermined ratio,'in combination, a plurality of motors each provided with separately-excited main field windings,

difierential-field and cumulative field windings, and means for individually'exciting the fieldwindmgs of each motor, said ex iting means being arranged to respond to the leads carried by the motors to cause an increase in thejexcitation of the cumulative-fieldwind ing ofthe motor exceeding its proportion of the load and an increase in the excitation of the diiierential-field winding of the other motor to reestablish the load ratio.

4. In a power system, in combination, a plurality of motors disposed to divide'a com" mon load in a predetermined ratio, the motors being provided with separately-excited main field windings which determine the load ratio and auxiliary-field windings, the auxillary-field wmdmgs of each motor being dilterentially arranged to balance when there is proper division of load, and sepaf rate exciters for the. auxiliary-field windings,

'mon load in a predetermined ratio,'each motorbeing provided with sep arately-excited main and auxiliary-field windings, the auxil iary-field windings of each motor being differentiallydisposed, and current responsiv means for exciting the auxiliary windings oi the motors, said exciting means being disposed to effect a balance betweenthe auxiliary windings of each motor when the motors divide the load in the predetermined ratio and to unbalance the auxiliary field windings to correct for a division of load which is not in accordance with said load ratio.

6. Ina power system, in combination, a plurality of motors disposed to dividea common load ina predetermined ratio, the motors being provided with separately-excited main ,fie'ldwindings and auxiliary-field windings,

the auxiliary-field windings of each motor being di'fierentially disposed, and 'electro-responsive'means for individually exciting the auxiliary-field windings of the motors, said .electro-re'sponsive exciting meansbeing disratio by eiiecting an unbalance, between the auxiliary field windings of each motor.

7. In a power system for motors dividing the load in a predetermined ratio, in combination, a plurality of motors, each provided with main, difierential compounding and cumulative compounding field windings,

and separately-driven generator means for exciting the'field windings of each motor,

said exciting means being disposed to respond to the loads carried by their respective motors to cause an increase in the excitation of the cumulative compounding-field and a decrease in the excitation of the differential compounding-field of a motor carrying a load greater than its predetermined proportion, and an increase in the excitation of the differential field and a decrease in the excitation of the cumulative field of the motor carrying a load less than its proportion to reestablish a predetermined ratio in the'loading of the motors. V

8. In a motor-control system, in combination, a plurality of motors provided with main and series-compounding field windings disposed to drive a common load, a source of power for the motors, said motors being connected in parallel-circuit relation to the power source, means for controlling the excitation of the main field windings to vary the relative motor speeds, and eletrical means associated with, and responsive to, the load current of each motor for separately controlling the relative excitation of the seriescompounding field windings of each motor, thereby to automatically vary the strength of the total field flux produced by the main field and compounding-field windings to maintain any predetermined speed relation between the motors, as established by the initial main field excitation.

9. In a motor-control system, in combination, a plurality of motors provided with main and series compounding-field windings, a source of power for the motors, said motors being connected in parallel-circuit relation to the power source, manual means for varying the main field excitation of the motors to establish a predetermined speed relation between the motors, rotatable means associated with each motor toindividually vary the excitation of the series-compound'ingfield windings in accordance with the loads on the respective motors to'produce a total variation of the field flux of both motors in proportion to the variation in load, and means for driving said rotatable means at a substantially constant speed.

10. In a system for controlling the speed relation between a plurality of individual motors, in combination, a source of power for the motors, said motors being connected in parallel circuit relation to the power source, each motor being provided with main field windings and cumulative and differential compounding-field windings, and separate means responsive to a change in load ratio for controlling the excitation of said compounding-field windings, tovary the field excitation differentially with respect to the loads there by to correct for load unbalance.

11, In a power system, in combination, a plurality of motors, a source of power for the motors, said motors being connected in parallel-circuit relation to the power source, each motor having a main field winding and cumulative and differential compoundingfield windings, means for varying the excitation of the main field Winding to. predetermine-the load ratio ofthe motors, and means including separately-driven exciters responsive to the motor loads for controlling the relative excitation of the compounding-field windings to maintain the predetermined load ratio.

'12. In an electric drive for rolling mills provided with a plurality of reducing rolls, in combination, a plurality of motors individually connected to the reducing rolls, a source of power for the motors, the motors being provided with separately-excited main field, cumulative and differential compounding field windings, means for controlling the relative excitation of the main field windings to establish a predetermined load ratio, a pair of exciters for each motor, one of each pair being connected to the cumulative compounding-field winding of its motor, the other to the differential compounding-field winding of another motor, said exciters being disposed to balance the excitation of the compounding-field windings of each motor when the load is dividing in said predetermined ratio and to act in response to changes in motor-load currents, thereby to unbalance the excitation of the motors in proportion to the unbalance in load to correct the unbalanced load condition.v Q

13. In an electric drive for rolling mills provided with a plurality of reducing rolls, in combination, a plurality of motors individua'lly connected to the reducing rolls, a

source of power for the motors, the motors bein provided with separately excited main field, cumulative and differential compounding-field windings, means for controlling the relative excitation of the main field windings to establish a predetermined load ratio, an exciter for each compounding-field winding, said exciters having field windings connected in series with the roll motors, each differential compounding-field Winding be- 'ig cross connected to an exciter of another motor to effect a balance of the fields produced by the compounding-field. windings of each motor when the motors are properly dividing the load and to produce equal and opposite variations in the field excitation of the respective motors, thereby to produce a" total corrective unbalance between the fields of the motors double in proportion to the lead unbalance.

14-. The combination with a rolling mill provided with a plurality of reducing rolls, indiv dually operating the rolls, a generator for supplying of a plurality of motors for power to the motor means including a master switch for controlling the direction and value of'the generator voltage to control the speed and direction of operation of the motors, said motors being provided vith separately-excited main field windings and cumulative and differential com pounding field windings, a series eXcit-er for each compounding field winding associated witheach motor, one of the compounding field windings of each motor being connected to one of the es:- citers associated with the other motor to provide for maintaining a predetermined load ratiobetween the motors, and means responsive. to the operation of the master switch for rendering the series eXciters effective to maintain load division when the motors are operating in either direction.

15. In combination, a plurality of cooperating roll members, a motor connected to each roll member, each of said motors being provided with a main field winding and a pair of diilerentially-disposed auxiliary field windings, a source of excitation current for the main field windings, and separatelydriven exciter means associated with each motor disposed to automatically vary the excitation of the auxiliary field windings in accordance with the loads carried by their respective motors. V

16. In combination, a plurality of cooperating-rolls, a motor for actuating each roll, said motors being provided with main field windings and cumulative and differential auxiliaryfield windings, a source of power for the motors, means for varying the excitation of the main field windings, means associated with each motor responsive to the load currents of the motors for simultaneously controlling the excitation-of the aux,- iliary field windings of all the motors, and means operable to render said last named means efiective regardless of the direction in which the motors are operated.

1'7. In combination, a plurality of roll members, a motor for actuating each roll, a source of power for the motors, means "for manually controllin one component of excitation of the motors, and separately-driven generators associated with each motor for simultaneously controlling another component of excitation of their respective mot rs and of the other motors,,said generators being disposed to function in accordance with the load currents of their respective motors thereby to provide for automatically maintaining a predetermined load ratio'between the motors and a predetermined speed ratio between the rolls.

- 18. In combination, a plurality of roll members, a motor for actuating each roll, a sourceof power-for the motors, means for manually controlling one component of excitation of the motors, a plurality of eXciters.

associated with each motor, said exciters havin field windings connected in series-circuit re ation with the armatures of their respective motors, means for driving the exciters at a substantially'constant speed, and means witha separately-excited main field winding and cumulative and differential compounding field windings, a source of power for the motors, means for controlling the excitation of the main field windings, means for con necting one of the compounding field windings of each motor in series circuit relation with the armature of the'other motor, and means associatedwith each motor disposed to develop a voltage proportional to the load current of their respectivemotors for controlling the excitation of the other compounding field windings.

20. In combination, a pair of cooperating roll members, a motor for actuating each roll member, each of said motors being provided with a separately-excited main field winding and cumulative and difierentialcompounding fieldwindings, a source of power for the motors, means for controlling the excitation of the main field windings, means for rendering the excitation of one compounding field winding of each motor dependent upon the loadof the other motor, and a separatelydriven exciter associated with each motor disposed to control the excitation of the other compounding field windings of their respective motors, said exciters having field windings disposed to be energized in, accordance with the load currents of their respective motors.

21. In combination, a plurality of-coopcrating roll members, a plurality of motors having main, cumulative compounding and differential compounding field windings for individually actuating the roll members and dividing the total load in a predetermined ratio, a source of power for the motors,

manually-operable means for controlling the excitation of the main field windings to (1011-.

trol one component ofthe excitation of the motors, means associated with each motor disposed to develop voltages proportional to the load currents of the motors for controlling the excitationot the compounding field windings to automatically vary another component of the excitation of the motors to effect relative variations of the total excitation of the motors proportional to thedegree of departurefrom a predetermined load ratio thereby to maintain predetermined speed and load ratios. V

In testimony whereof, I have hereunto subscribed my name this 14th day, of June 1929. r HAROLD E, STOKES. 

